This invention relates to a monolithic semiconductor laser structure and, more particularly, to an independently addressable, vertical cavity surface emitting laser ("VCSEL") in the blue wavelength range.
Monolithic solid state semiconductor lasers are very desirable light sources for high speed laser printing, optical fiber communications and other applications. Recently, there has been an increased interest in vertical cavity surface emitting lasers although edge emitting lasers are currently used in the vast majority of applications. A common laser structure is a so-called "edge emitting laser" where light is emitted from the edge of a monolithic structure of semiconductor layers. A laser structure is a "VCSEL" where the light is emitted from the surface of the monolithic structure of semiconductor layers.
A reason for the interest in VCSEL's is that edge emitting lasers produce a beam with a large angular divergence, making efficient collection of the emitted beam more difficult. On the other hand, not only does the beam of a VCSEL have a small angular divergence, a VCSEL emits light normal to the surface of the wafer.
Prior art quantum well vertical cavity surface emitting lasers can emit light in the wavelength range of about 600 to 650 nanometers (the red range) or in the range above 650 nanometers (the infrared range). However, there are important applications for laser devices that emit light in the wavelength range significantly around 400 nanometers (the blue range).
One ongoing problem with blue lasers in general is that gallium nitride and its alloys with indium and aluminum are used as the semiconductor layers in the blue laser structure. GaN itself cannot be used as a practical substrate. The problem is the lack of a suitable, lattice-matched substrate for the GaN semiconductor layers. Without a substrate with a lattice constant close to that of GaN, there will be a high density of extended defects in the nitride layers. An active layer of GaN and its alloys on a poorly lattice matched substrate will emit coherent blue light poorly and inefficiently, if at all.
The most commonly used and most readily available substrate is sapphire (Al.sub.2 O.sub.3). Sapphire, however, has a similar crystal structure to GaN. An approximate epitaxial relationship between a sapphire substrate and a GaN alloy semiconductor laser structure can be obtained by matching specific crystallographic directions. However, a lattice mismatch of approximately 15 percent results from such a semiconductor structure.
It is an object of the present invention to provide an independently addressable, monolithic vertical cavity surface emitting laser structure for emitting light in the 390 to 430 nanometer range (the blue range).
It is another object of the present invention to provide a suitable lattice matched substrate for a blue VCSEL.